1. Field of the Invention
The present invention relates to a graphics drawing device, and more particularly, to a graphics drawing device for drawing a main graphic and a derivative graphic derived therefrom.
2. Description of the Related Art
In a graphics drawing device such as a car navigation system, information stored in a CD-ROM (Compact Disk Read Only Memory), a DVD (Digital Versatile Disk) or the like needs to be read out so that graphics may be drawn and displayed on screen.
There are generally two drawing methods for generating an image (image of a crossing with an underpass) as shown in FIG. 13, for example. The first method is to draw white line segments and black line segments in the order mentioned below. To explain the method with reference to the numerals shown in FIG. 14, first, black line segments (8), (7) and (6) are drawn, and then a white line segment (4) with a smaller width is drawn on the black line segment. Further, a black line segment (5) and then white line segments (3), (2) and (1) are drawn, whereupon the drawing is completed.
FIG. 15 shows an image obtained in the case where the line segments are drawn in order of (8), (4), (7), (3), (6), (2), (5) and (1). Thus, with the first method, if the line segments are drawn in different order, the desired image shown in FIG. 13 cannot be obtained.
The other method is to automatically set the drawing order based on information about the depth direction of graphics (in this instance, line segments) to be drawn by the graphics drawing device. With this method, it is unnecessary to take the drawing order into account, unlike the aforementioned method.
In the former drawing method, the drawing order must be determined in advance. A problem therefore arises in that an additional load is imposed on a device (e.g., a CPU (Central Processing Unit)) other than the graphics drawing device.
In the latter method, on the other hand, the load on the other processing device does not increase, but information indicating the depth direction of graphics also needs to be supplied to the graphics drawing device, giving rise to a problem that the amount of information transferred from the CPU to the graphics drawing device increases.